As methods for preparing compound metal oxides, there have been known processes in which
(1) Oxides, carbonates or oxalates of the respective component metals are physically mixed in the state of powder, whereafter the mixture is fired and pulverized;
(2) Water-soluble salts such as nitrates of the respective component metals are dissolved in water to make a homogeneous solution, the metals are coprecipitated as insoluble hydroxide, carbonate, oxalates, etc. by addition of hydroxide ions, carbonate ions, oxalate ions, etc. the precipitate is collected and fired into a compound metal oxide, etc.
In process (1), powders are mixed and fired. Therefore, the reaction proceeds from the contact boundaries of the powder particles and there remain some unreacted portions. Thus the product often deviates in composition from the intended exact composition.
In process (2), the condition of precipitation differs from metal to metal. Therefore, a precipitate containing a rare earth metal, an alkaline earth metal and copper, for instance, in the content ratio exactly the same as the content ratio in the solution is not always formed. Therefore, a compound metal oxide of a composition different from that intended is inevitably formed.
The above-mentioned problems can be solved by heating for reaction a composition comprising an alkoxide of a rare earth metal, an alkoxide of an alkaline earth metal and an organic acid salt, a .beta.-diketone complex, a .beta.-ketoester complex or a Schiff base chelate complex of copper, optionally containing an organic solvent, and thermally decomposing the reaction product.